Announcement: Upgrade to macOS Mojave
With features like Dark Mode, Stacks, and four new built-in apps, macOS Mojave helps you get more out of every click. Find out how to upgrade to macOS Mojave > https://support.apple.com/macos/mojave
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Do not buy that Mac until its owner properly prepares it for sale. Refer to What to do before you sell or give away your Mac - Apple Support.
Assuming that Mac's hardware works properly, preparing it for sale is a very simple task. If the Mac's hardware is not working properly, you might as well buy a new Mac for what it would cost to fix it.
Buying a used Mac is a task best left to very experienced Mac owners. If you want to save a few dollars, buy one from the Apple Refurbished and Clearance page.
Question: getting ready to buy a MacBook Pro